1. Field of the Invention
The present invention relates to an ingress noise control system and an ingress noise blocking device which are used in a cable system.
A cable system is known that provides two-way communication using different frequency bands for transmission of upstream and downstream signals. In such a system, upstream signals from many users (terminal equipments) are combined for transmission to a head end (center). At this time, noise from each user (terminal equipment) is also coupled into the signal stream, causing S/N degradation. There is therefore a need to reduce this noise.
2. Description of the Related Art
FIG. 1 is an explanatory diagram of a CATV system, in which reference numeral 51 is a center, 52 is a two-way amplifier, 53-1 and 53-2 are distributors, and 54-11, 54-12, . . . , 54-21, . . . are terminal equipments. The CATV system can be built to provide various communication services such as on-demand CATV, cable telephony, Internet connections, etc. via coaxial cable or optical fiber transmission line.
The two-way amplifier 52 is included to amplify signals attenuated in transmission, and a plurality of such amplifiers are provided in the CATV system, depending on the transmission distance. The distributors 53-1 and 53-2 connect between the trunk transmission line and the plurality of terminal equipments, and each distributor employs, for example, an eight-branch design. The available spectrum is divided into two frequency bands, for example, a frequency band of 5 MHz to 55 MHz for upstream signals and a frequency band of 70 MHz to 770 MHz for downstream signals, enabling the upstream and downstream signals to be separated from each other according to the frequency band. For the modulation scheme, it is common to employ phase modulation such as BPSK, QPSK, QAM, or the like. Transmission rates heretofore known include 160 kbps, 320 kbps, 640 kbps, 2560 kbps or 256 kbps, 1544 kbps, 3088 kbps, etc.
Downstream signals from the center (head end) 51 are distributed through the distributors 53-1 and 53-2 to the respective branch transmission lines for transmission to the terminal equipments 54-11, 54-12, . . . , 54-21, . . . On the other hand, upstream signals from the respective terminal equipments are transmitted toward the center 51 through the opposite path to that for the downstream signals. In that case, the upstream signals are combined together in the distributors.
The upstream signals are transmitted from the terminal equipments toward the center 51 via the distributors 53-1 and 53-2; at this time, noise components from each terminal equipment are added in the signal stream. This added noise is known as ingress noise. If the ingress noise is due to induction, etc. from radio circuitry, it is known to provide a system that transmits the upstream signal by converting its frequency to a frequency different from that of the radio circuitry.
However, when a user terminal equipment is removed because the user is moving or for other reason, the transmission line will be left open, allowing a high level of noise to enter. This kind of noise cannot be reduced by frequency conversion. Furthermore, since the signal component is zero and only the noise component is transmitted, the noise is sequentially summed through the distributors 53-1 and 53-2, increasing the ingress noise and thus significantly degrading S/N; this can render the discrimination of upstream signals impossible at the center and can lead to a system down situation.